The present invention relates to a method for fabricating a semiconductor device for use as, e.g., a short-wavelength light-emitting diode, a short-wavelength semiconductor laser, or a high-temperature and high-speed transistor.
A nitride semiconductor which has a large optical band gap (e.g., GaN has an optical band gap of about 3.4 eV at a room temperature) has been conventionally used as a material for implementing a visible light-emitting diode which emits light in a relatively short wavelength region such as green, blue, or white light or a short-wavelength semiconductor laser which would be used for future high density optical disks. In particular, a nitride semiconductor has been used prevalently for the active layer of a light-emitting diode. As a light source for a read/write operation to a high-density optical disk, the commercialization of a blue or blue-purple laser has been in strong demand.
It is general practice to form each of nitride semiconductor layers composing a device on a sapphire substrate having a principal surface substantially coincident with the (0001) plane by typical metal organic chemical vapor deposition. In the case of fabricating a semiconductor laser, it is necessary to form cleavage planes serving as mirrors of the cavity at the edge portions of the semiconductor laser structure of the nitride semiconductor layers with a waveguide structure and electrodes. However, it has been difficult to cleave the entire substrate since the crystal structure of the sapphire substrate has rotated by 30° from that of the nitride semiconductor in the c plane ((0001) surface)) and sapphire is hard to be cleaved. This has prevented the formation of satisfactory resonator surfaces (mirrors) and made it difficult to achieve a high performance semiconductor laser, especially with a low threshold current.
To solve the problem, there has been proposed a method of epitaxially growing nitride semiconductor layers, adhering the nitride semiconductor layers to a recipient substrate made of a material that allows successful formation of cleavage planes of the nitride semiconductor layers thereon, separating the nitride semiconductor layers and the sapphire substrate from each other, and thereby cleaving the nitride semiconductor layers or the recipient substrate. In accordance with the method, the separation of the nitride semiconductor layers and the sapphire substrate is accomplished by irradiation of a laser beam from the back surface of the sapphire substrate and thereby decomposing or fusing a GaN layer and the like present at the interface with the sapphire substrate. The method uses, e.g., an Si (001) substrate as a recipient substrate. By bringing the cleavage plane of Si and the cleavage plane of GaN into parallel relation upon adhesion, the nitride semiconductor layers can be formed with two flat resonator surfaces parallel to each other. This allows such a high performance semiconductor laser with a lower threshold current and a longer lifetime.
A description will be given to a method for fabricating the aforementioned nitride semiconductor device. FIGS. 16A to 16D are cross-sectional view illustrating the conventional method for fabricating a nitride semiconductor device.
First, in the step shown in FIG. 16A, an epitaxially grown layer 103 having a multilayer structure including a GaN layer, an AlGaN layer, and an InGaN layer and having a pn junction portion is formed by, e.g., metal organic chemical vapor deposition (MOCVD) on a sapphire substrate 101 (wafer). In the case of fabricating a semiconductor laser, a waveguide structure has been incorporated into the epitaxially grown layer 103 by using a regrowth technique, selective etching of the epitaxial layer, or the like.
Next, in the step shown in FIG. 16B, the epitaxially grown layer 103 is adhered to an Si substrate 104 having a principal surface substantially coincident with the (001) plane. In the step shown in FIG. 16C, the back surface of the sapphire substrate 101 is irradiated with a KrF excimer laser beam (at a wavelength of 248 nm).
FIG. 17 is an energy band diagram showing band states in the sapphire substrate 101 and in the GaN layer included in the nitride semiconductor layers. Since the band gap (optical band gap) of the sapphire substrate 101 is large, as shown in the drawing, the output of the KrF excimer laser beam is not absorbed by the sapphire substrate 101. Because the band gap (optical band gap) of the GaN layer is small, the laser beam used for irradiation is absorbed by the GaN layer so that, if the power of the laser is extremely high, the energy of the laser beam is consumed to break chemical bonds so that the bonds in the GaN layer are broken in the vicinity of the interface with the sapphire substrate 101.
Consequently, the sapphire substrate 101 and the epitaxially grown layer 103 are separated from each other, as shown in FIG. 16D. Thereafter, the process of forming an electrode which comes in contact with the epitaxially grown layer 103 on the Si substrate 104, cleaving the substrate (in the case of fabricating the semiconductor laser), and the like is performed. In the case of fabricating the semiconductor laser, the epitaxially grown layer 103 and the Si substrate 104 are adhered to each other such that the <11-20> direction of the GaN layer and the <110> direction of the Si substrate are in parallel relation for easy cleavage.
The foregoing fabrication method allows formation of flat resonator surfaces of the semiconductor laser. Since the nitride semiconductor layers are adhered to the Si substrate with superior heat dissipation, the semiconductor laser is expected to have a longer lifetime.
However, the foregoing method for fabricating a nitride semiconductor device has the following problems.
In the step shown in FIG. 16C, the irradiation with the KrF excimer laser beam increases the probability of a crystal defect or a crack occurring in the region of the GaN layer adjacent the interface between the epitaxially grown layer 103 and the sapphire substrate 101. This narrows down an optimum range in which the power of the KrF excimer laser. If the epitaxially grown layer is as thin as about 4 μm, a crystal defect and a crack extend the surface of the epitaxially grown layer so that it is necessary to form the nitride semiconductor layers (epitaxially grown layer) having a total film thickness as large as about, e.g., 10 μm. If the nitride semiconductor layers on the sapphire substrate are increased in film thickness, however, the bowing of the entire wafer during cooling after epitaxial growth becomes conspicuous due to the different thermal expansion coefficients of the sapphire substrate and the nitride semiconductor layers so that it is difficult to adhere the flat recipient substrate to the wafer.